The synthesis and assembly of two specialized brain membranes is under investigation. The kinetics of incorporaton of radioactive amino acids and sugars into mylein proteins and glycoproteins, and of radioactive lipid precursors into myelin lipids, will be quantitated in the brains of developing rats. The flow of individual radioactive proteins and lipids through dense mylelin "precursor fractions" will be compared in normal developed in normal developing animals and in animals made hypothroid or animals treated with triethyltin during early development. The reversibility, or lack thereof, of any myelination deficit will be investigated. In other studies radioactive protein or lipid precursors will be stereotaxically injected into certain brain nuclei (e.g., lateral geniculate body, substantia nigra) and transport of locally synthesized proteins to the nerve terminals at the projection site (visual cortex, striatum) determined. Proteins in differen transport classes (fast, intermediate, slow) will be characterized by several electrphoretic procedures. The question under investigation in these studies is whether neurons in anatomically and functionally discrete classes still utilize the same classes of proteins to support their nerve ending activities.